A room audio system, such as a conference phone, can be used to conduct audio meetings between groups of participants that are remote with respect to each other. These devices allow the meeting participants to position themselves in a range of positions and orientations within a conference room or around a conference table in order to effectively participate in a conference call.
Among other things, conference phones or conference systems typically integrate loudspeakers into a housing with some number of microphones. Positioning a loudspeaker proximate to microphones creates a number of problems with respect to the capture and processing of audio signals (voice signals) from the local environment. The proximity of a loudspeaker to a microphone results in the microphone capturing energy from the loudspeaker (called acoustic coupling . . . far-end voice) which is then sent back as an acoustic echo to a far-end audio system where the participants hear their own voices as echo. This acoustic echo is distracting and denigrates the quality of an audio conferencing session. While it is possible to remove a certain amount of this acoustic echo in a microphone signal (maybe 25-30 db of acoustic echo energy) by applying acoustic echo cancellation (AEC) methods to the signal, the resulting audio signal can still include some acoustic echo energy.
One design technique that is typically used to mitigate the effects of acoustic coupling between a loudspeaker and microphone is to place the microphones as far away from the loudspeakers as is possible, and to position the microphones so that their positive polar pattern is oriented away (faces away) from the direction of loudspeaker energy waves. Typically, directional microphones are employed that exhibit a cardioid polar response pattern, where one side of the microphone is much less sensitive to acoustic energy than the other side. Moving microphones away from a loudspeaker and employing directional microphones further reduces the acoustic coupling between microphone and a loudspeaker proximate to them. A range of microphone polar response patterns are illustrated with reference to FIG. 1. Another advantage to the use of directional microphones is that they operate to pick up more of the direct sound waves and less of the reflected sound from the walls and ceiling than an omni-directional microphone. This makes the voices sound less reverberant and results in better intelligibility than with a single or multiple omnidirectional microphones. If directional microphones are used, typically a switching algorithm selects the microphone with the highest energy and mutes the remaining microphones.
Loudspeakers associated with audio conference systems are generally positioned at a central location with respect to the microphones comprising the audio system. Additionally, the microphones are typically located at the end of microphone arms that extend radially away from the central loudspeaker. The length of these microphone arms is dictated by the amount of echo return loss needed to provide a microphone signal that, after being processed, is relatively free from far-end voice energy. Alternatively, the entire body of an audio conference system can be extended laterally from a central loudspeaker location, and one or more microphones can be positioned at the outside radius or edges of the lateral body extension.
As described above, the directional microphones comprising an audio conference system are typically placed at the distal ends of these arms with respect to a central audio system location (loudspeaker position), and the microphones can be placed in a specially designed microphone housing that maximizes their exposure to a conference room environment while minimizing their exposure to loudspeaker energy. Such an audio conference phone arrangement is illustrated with reference to FIG. 2. As the directional microphones operate according to a pressure gradient difference between their front and back (as determined by their polar response pattern), anything that distorts this pressure gradient (reflected energy of any type) tends to reduce the directional characteristics of a microphone. It is critical to the operation of a directional microphone that the design of this microphone housing and placement of the microphone within the housing is affected as little as possible by reflected energy from the housing walls. Consequently, microphone housings are designed without side wall surfaces and with a back wall (behind the microphone) that is at least 2.5 cm away and sloped at an angle away from the microphone so that energy is reflected upward and away from the microphone.